1. Field of the Invention
The present invention relates to a taking optical system, and more particularly to a slim taking optical system suitable for use in a digital device equipped with an image input capability so as to be capable of capturing an image of a subject by means of an image sensor.
2. Description of Related Art
In recent years, more and more cellular phones and personal digital assistants (PDAs) have been becoming equipped with a digital still camera or digital video unit for capturing images. These digital devices are required to be compact so as to be easily portable, and are simultaneously required to capture images with high performance. For high-performance image capturing, image sensors for capturing images have been made increasingly high-definition and, with this trend, the image sensing element that forms each pixel has been made increasingly small. Even then, image sensors themselves have inevitably been becoming increasingly large, requiring increasingly large image sizes, and moreover, as increasingly high definition is sought, accordingly high resolution has been sought in image sensors. Conventionally used as taking optical systems are coaxial optical systems of a so-called straight type. Seeking larger image sizes and higher resolution in this type of optical system, however, leads to an increased number of lens elements and an increased total length. This is contrary to the trend for compactness in cellular phones and PDAs. Thus, compactness and slimness need to be pursued with optical systems of an other-than-straight type.
As taking optical systems of an other-than-straight type, there have conventionally been known optical systems that employ a prism having a reflective surface. In this type of optical system, compactness and slimness are pursued by bending the optical path with a reflective surface. The optical path is bent, for example, by bending it a plurality of times within a single prism, or by bending it a plurality of times with a plurality of prisms. For example, Patent Publication 1 listed below proposes a taking optical system that pursues overall slimness by using two prisms, and Patent Publication 2 listed below proposes a taking optical system that pursues cost reduction by using a single prism.                Patent Publication 1: JP-A-2003-084200        Patent Publication 2: JP-A-H11-023971        
The taking optical system disclosed in Patent Publication 1 uses two prisms, resulting in high cost. Moreover, an aperture stop is disposed between the two prisms, increasing the number of components and thus further increasing the cost. Furthermore, inaccurately positioning the aperture stop leads to poor optical performance. The taking optical system disclosed in Patent Publication 2 uses only one prism, advantageously in terms of cost reduction. Moreover, an aperture stop is disposed inside the prism, promising further cost reduction. However, although slimness is achieved to some extent, higher definition and wider angles of view cannot satisfactorily be coped with. Specifically, unless the prism is properly designed and arranged, it is not possible to achieve satisfactory slimness, nor to obtain high performance.